Difference between revisions of "cpp/thread/notify all at thread exit"

Latest revision as of 23:39, 13 March 2024

notify_all_at_thread_exit provides a mechanism to notify other threads that a given thread has completely finished, including destroying all thread_local objects. It operates as follows:

• Ownership of the previously acquired lock lk is transferred to internal storage.

• The execution environment is modified such that when the current thread exits, the condition variable cond is notified as if by lk.unlock();
  cond.notify_all();.

The implied lk.unlock() is sequenced after the destruction of all objects with thread local storage duration associated with the current thread.

If any of the following conditions is satisfied, the behavior is undefined:

• lk is not locked by the calling thread.
• If some other threads are also waiting on cond, lk.mutex() is different from the mutex unlocked by the waiting functions (wait, wait_for and wait_until) called on cond by those threads.

[edit] Notes

An equivalent effect may be achieved with the facilities provided by std::promise or std::packaged_task.

The supplied lock lk is held until the thread exits. Once this function has been called, no more threads may acquire the same lock in order to wait on cond. If some threads are waiting on this condition variable, ensure that the condition being waited for is satisfied while holding the lock on lk, and that this lock is not released and reacquired prior to calling notify_all_at_thread_exit to avoid confusion from spurious wakeups in other threads.

In typical use cases, this function is the last thing called by a detached thread.

[edit] Parameters

[edit] Return value

(none)

[edit] Example

#include <cassert>
#include <condition_variable>
#include <mutex>
#include <string>
#include <thread>
 
std::mutex m;
std::condition_variable cv;
 
bool ready = false;
std::string result; // some arbitrary type
 
void thread_func()
{
    thread_local std::string thread_local_data = "42";
 
    std::unique_lock<std::mutex> lk(m);
 
    // assign a value to result using thread_local data
    result = thread_local_data;
    ready = true;
 
    std::notify_all_at_thread_exit(cv, std::move(lk));
 
}   // 1. destroy thread_locals;
    // 2. unlock mutex;
    // 3. notify cv.
 
int main()
{
    std::thread t(thread_func);
    t.detach();
 
    // do other work
    // ...
 
    // wait for the detached thread
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{ return ready; });
 
    // result is ready and thread_local destructors have finished, no UB
    assert(result == "42");
}

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also